The present invention relates to a glow plug for promoting ignition and combustion of fuel, an ion current detector using the glow plug and a method of manufacturing the glow plug.
From the standpoint of the environmental protection, further reduction of the discharged quantity of exhaust gas or soot has been recently required in not only gasoline engines but also Diesel engines. To meet the requirements, consideration has been given to improvements on various points such as improvements of engine, a reduction in emission gas by the post processing (using a catalyst or the like), improvements in characteristics of fuel or lubricating oil and improvements of engine combustion control systems.
In connection with the above countermeasures, it is required to detect engine combustion conditions during engine operation. Such engine combustion conditions are to be detected by measuring cylinder internal pressure, combustion light, ion current and such. Of all the measuring methods for detecting the engine combustion conditions, the ion current measurement has been considered to be highly useful because it can be used for directly observing a chemical reaction resulting from the engine combustion, and therefore various types of ion current detecting methods have been proposed.
Japanese Patent Laid-Open Application (JP-A) No. 7-259597 discloses a method for detecting ion current (ionization degree of fuel gases) due to combustion of fuel by a sleeve-like electrode attached to a mounting seat for a fuel injection nozzle, the sleeve-like electrode electrically insulated from the injection nozzle and a cylinder head of the engine, and connected to an external detection circuit.
U.S. Pat. No. 4,739,731 discloses a sensor provided with a ceramic glow plug for detecting ion current (conductivity of ionized fuel gases). In this technique, an electric conductive layer made of platinum is formed on a surface of a heater (heating element) of the ceramic glow plug, and electrically insulated from a combustion chamber and a glow plug clamping fixture. An external power source (for 250-volt DC voltage) is provided for applying the voltage to the electric conductive layer to detect ion current resulting from the fuel combustion.
In typical ion current detectors with a glow plug having such an ion current detecting function, ignition and combustion of fuel are generally promoted by a heating action of the heating element when the engine starts at low temperature. In this case, such a heating state of the heating element usually continues after warm-up of the engine has been completed until the combustion is stabilized (generally, referred to as xe2x80x9cafterglowxe2x80x9d). After completion of the afterglow, the heating action of the glow plug is stopped and the processing step of detecting ion current is started.
However, the following drawbacks are present in the above conventional techniques. With the former technique (JP-A No. 7-259597), there is a need for ion current detection to provide a sleeve-like electrode insulated from the other portions, and this forces complicated work in preparing electrode materials and machining the electrode. Such an electrode for ion current detection is thus very expensive, and besides, becomes unusable earlier because of short-circuit between the electrode and the fuel injection nozzle or the cylinder by carbon generated in the combustion chamber.
With the latter technique (U.S. Pat. No. 4,739,731), since the electrode for detecting ion current is provided on the heating element, and the electrode and the heating element are connected to different power sources through individual electric circuits, respectively, the circuit structure is complicated. In addition, since a large amount of expensive noble metal such as platinum is needed for ensuring heat and wear resistance of the electrode, the glow plug itself becomes very expensive. Further in this sensor, the electrode is almost completely exposed into the combustion chamber and the space between the housing and the electrode is narrow. For this reason, there is a danger that the electrode is shorted to the ground and the housing is made conductive due to adhesion of carbon to the electrode surface, resulting in an error in detecting ion current.
Existing ion current detectors display only a heating action and cannot detect ion current during the afterglow period. Since in this period any result of ion current detection can not be used for performing combustion control, the combustion cannot be controlled optimally. Stated more specifically, it is difficult to control the combustion optimally during the afterglow period because such a result of ion current detection cannot be used in individual combustion operations, e.g., for performing feedback control of ignition stage and flame failure detection.
When using the above conventional glow plug, carbon adheres to the circumference of the ceramic heating portion to reduce insulation resistance between the exposed electrode for ion current detection and the grounded portion (plug housing and cylinder head) insulated from the electrode. In this case, a flow of leakage current may be created through the adhered carbon even if no ion is derived from the combustion gases. When this happens, the ion current detected shows a waveform different from a desired one due to occurrence of the leakage current, and such an incorrect detection result causes deterioration in the accuracy of ignition stage and flame failure detections. The electric insulation between the exposed electrode and the ground portion is dependent on pressure in the combustion chamber. Especially, in the engine compressing process the insulation resistance drops and the leakage current becomes easy to flow.
Also when using the glow plug, a sharp temperature change runs the danger that the ion current detecting electrode is broken by thermal vibration. Since a large amount of expensive noble metal such as platinum is needed for ensuring heat and wear resistance of the electrode, the glow plug itself becomes very expensive.
Further, since the ion current detecting electrode supported at the tip of the glow plug directly touches a flame having a high temperature, stresses tend to be concentrated in the neighborhood of the ion current detecting electrode and could damage the ceramic glow plug such as to crack it.
Therefore, it is an object of the present invention to provide a glow plug capable of detecting ion current precisely with a simple structure, an ion current detector using the glow plug, and a method of manufacturing the glow plug.
Another object of the present invention is to provide an ion current detector capable of detecting ion current precisely even for a period of glow of the glow plug and hence maintaining proper combustion of fuel based on the detection results.
Still another object of present invention is to provide an ion current detector capable of detecting ion current precisely and hence performing precise control of individual processings such as ignition stage detection and flame failure detection based on the detection results.
Yet another object of the present invention is to provide a relatively inexpensive glow plug having excellent durability, which can detect ion current precisely without any trouble from carbon adhesion and any damage to the ion current detector.
Yet another object of the present invention is to provide a glow plug having excellent durability without suffering any damage such as a crack and showing ease of manufacture, and a method of manufacturing the same.
The term xe2x80x9cion currentxe2x80x9d used here means current passing through ionized fuel gases in a combustion chamber. The ion current detecting electrode may be referred to as the ion current detecting electrode.
To achieve the above objects, a glow plug according to the invention as claimed in claim 1 comprises a heat resisting insulator, a heating element embedded in the heat resisting insulator and energized through a pair of lead wires, and an ion current detecting electrode embedded in the heat resisting insulator with a portion of the ion current detecting electrode exposed to a flame produced in a combustion chamber so that an ionization state in the flame can be detected.
In this case, the heating element of the glow plug acts to promote ignition and combustion of fuel in the combustion chamber by a heating action of the heating element. The ion current detecting electrode embedded in the heat resisting insulator detects the ionization state in the combustion flame. When detecting ion current, the ion current detecting electrode and the inner wall of the combustion chamber adjacent to the ion current detecting electrode form two electrodes for capturing positive and negative ions existing therebetween during fuel combustion. According to the glow plug, the ion current can be detected precisely with very simple structure, and information detected can be effectively used for combustion control. Further, since the glow plug is given an ion current detecting function, an inexpensive ion sensor can be provided.
Since the glow plug of the invention is constructed such that the majority of the ion current detecting electrode is embedded in the heat resisting insulator except only a portion exposed to the outside, an amount of carbon adhered to the outer surface of the glow plug cannot establish an electric connection between the electrode and a housing (grounded side) to cause error detection of the ion current that may occur in the prior art (U.S. Pat. No. 4,739,731). The exposed portion of the ion current detecting electrode is preferably provided at the tip of the glow plug so that the exposed portion and the housing (inner wall side of the combustion chamber) will be separated as far from each other as possible.
Although it is considered that some carbon adheres to the outer surface of the glow plug during operation, the carbon adhered is burnt off by a heating action of the heating element 7 (e.g., due to glowing when the engine starts at low temperature). As a result, the glow plug can maintain its performance in detecting ion current for long periods.
Further, since the heating element itself is embedded inside the heat resisting insulator, it never change its heating characteristics due to lowering of resistance or the like to maintain high heating performance for long periods. In other words, since such a construction could resist oxidation to wear the heating element, the sectional area is kept constant and the resistance does not vary. The construction also avoids damaging the heating element under thermal action such as thermal shock in the combustion chamber.
In the glow plug of the invention, the heating element and the ion current detecting element is constructed as follows. The invention as claimed in claim 2 recites that the heating element and the ion current detecting electrode is electrically connected to each other. Stated more specifically, in the invention as claimed in claim 3 the heating element and the ion current detecting electrode are integrally formed, while in the invention as claimed in claim 4 lead wires reside between the heating element and the ion current detecting electrode. In all of claims 2 to 4, both the heating performance of the heating element and the performance in detecting ion current can be maintained for long periods as mentioned above. From the standpoint of the manufacturing process, it is considered that the invention as claimed in claim 3 shows the simplest way to manufacture the glow plug.
The glow plug as claimed in claim 5 is such that the heating element and the ion current detecting electrode are insulated from each other. Since the heating element and the ion current detecting electrode are energized through individual paths, the ion current detecting electrode can detect ion current synchronously with the heating action of the heating element (i.e., the combustion condition can be grasped constantly).
The invention as claimed in claim 6 recites that at least the portion of the ion current detecting electrode exposed to the flame is made of a conductive ceramic material. It is therefore possible to minimize oxidation wearing of the ion current detecting electrode even when it is exposed to hot combustion gases, and hence to further improve the durability of the performance in detecting ion current by the glow plug.
The invention as claimed in claim 7 recites that the heating element and the ion current detecting electrode are produced dividedly from each other by using mixtures having different components or different particle sizes. Such divided production can change the resistance between the heating element and the ion current detecting electrode to provide a glow plug (ion current sensor) according to the application. In the case where the result of ion current detection is used for flame failure detection, only the present or absence of ion current is required for the determination. In such a case, it is possible to increase the resistance of the ion current detecting electrode to a relatively large value, e.g., about 5 Mxcexa9 or less (1xcexa9 or so with the heating element). In the case where the result of ion current detection is used for ignition stage detection, it is desirable to reduce the resistance of the ion current detecting electrode as small as possible (500 kxcexa9 or less) since the leading edge of ion current must be detected for an instant.
Although the above description was made to the glow plug featured in that the glow plug itself can prevent the ion current detecting electrode and the housing (inner wall side of the combustion chamber) from conducting even when some carbon adheres thereto, the carbon may become adhered and accumulated during a long period of operation. To eliminate such a problem, an ion current detector as claimed in claim 8 features that the adhered carbon is removed without stopping the ion current detection by using the glow plug of claim 5 that can carry out the ion current detection by the ion current detecting electrode simultaneously with the heating action of the heating element. Specifically, the ion current detector comprises switching means for turning on or off the power supply to the heating element, leakage current detection means for detecting a leakage current flowing from the ion current detecting electrode in a predetermined stage before fuel combustion, and operation means for operating the switching means to temporarily energize the heating element when the leakage current detected is larger than a predetermined threshold.
When the carbon adheres to the outer portion of the glow plug in the combustion chamber, the exposed portion of the ion current detecting electrode and the housing side are electrically conducted to reduce the insulation resistance. Consequently, a leakage current flows and a desired ion current waveform cannot be obtained. As shown in FIG. 24B, the leakage current flows before its ion current waveform is obtained (before point A in FIGS. 24A and 24B). On the contrary, in the invention the carbon adhered state of the outer surface of the glow plug is estimated based on the leakage current, and if it is such a carbon adhered state, the adhered carbon will be burnt off by running the heating element hot. As a result, a desired waveform of ion current (e.g., the waveform shown in FIG. 24A) can be obtained at all times, and the detection result can be used for precise processings such as ignition stage detection and flame failure detection.
When the carbon adheres to the outer surface of the glow plug, the insulation resistance between the ion current detecting electrode and the housing side depends on the pressure in the combustion chamber. In the invention as claimed in claim 9, since the detection of leakage current is carried out when the pressure in the combustion chamber rises, the presence or absence of the leakage current can be detected securely. The timing period of the pressure rise corresponds to the compression stage in a Diesel engine, for example. The leakage current may also be detected in correspondence to the timing period of fuel injection into the combustion chamber. The timing period of fuel injection corresponds to a period that elapses between the moment the pressure in the combustion chamber of the Diesel engine rises and the moment just before the fuel burns. It is therefore possible to detect the leakage current more securely under such a condition that the carbon adhered.
The invention as claimed in claim 10 relates to a method of manufacturing the glow plug. In the manufacturing method, the heating element and the ion current detecting electrode are first produced, surrounded with the heat resisting insulator, and hot-pressed at a predetermined temperature. A portion of the heat resisting insulator is then cut to expose the ion current detecting electrode to the outside. According to the above manufacturing technique, the glow plug having such special structure can be made up without requiring any complicated process, thereby providing the glow plug having such an excellent ion current detecting function through the simple manufacturing method.
The invention related to the manufacturing method for the glow plug may be replaced by those as claimed in claims 11 and 12. The glow plug having the special structure and the excellent ion current detecting function as aforementioned can be manufactured even by the manufacturing methods of these claims without requiring any complicated process.
In the invention as claimed in claim 11, the heating element and the ion current detecting electrode are provided on a thin-plate like heat resisting insulation sheet to be wrapped around a rod-shaped heat resisting insulation solid-shaft. The heat resisting insulation sheet and the heat resisting solid shaft are heat-treated, and a portion of the heat-treated body of the heat resisting insulation sheet and the heat resisting solid shaft is cut so that the ion current detecting electrode will be exposed to the outside.
On the other hand, the invention as claimed in claim 12 is such that the heating element and the ion current detecting electrode are provided on certain one of plural layer members. Then the plural layer members are so superposed that the layer member having the heating element and the ion current detecting electrode thereon will reside in a central portion. After that, the plural layer members put on top of each other are heat-treated, and a portion of superposed layer members is cut so that the ion current detecting electrode will be exposed to the outside.
For solving the above problems, the present invention uses a glow plug having a heating element energized through a pair of conductive wires to generate heat, and the following ion current detector is constructed by using an ion current detecting function of the glow plug. In the glow plug, the conductive wire pair (lead wire pair) and the heating element are insulated from the grounded side such as a cylinder head.
An ion current detector as claimed in claim 13 includes switching means for switching over between a first state and a second state, in which the first state is for applying a supply voltage from a power source to the conductive wire pair, and the second state is for shutting the electric path between the conductive wire pair and the power source and applying the supply voltage between the heating element and a wall portion of a combustion chamber. Further, ion current detection means is provided for detecting ion current resulting from fuel combustion by using the voltage supplied from the power source in the second state.
In the first state, the supply voltage is applied from the power source to the conductive wire pair to run the heating element hot. This state corresponds to the state in which ignition and combustion of fuel is being promoted when the engine starts at low temperature. In the second state, the electric path between the conductive wire pair and the power source is shut and the supply voltage is applied between the heating element and the wall portion of the combustion chamber. This state corresponds to the state in which ion current is detected. The ion current is detected by the ion current detection means.
In such structure, the voltage application to the heating element is performed through the common conductive wire pair in both states, and the switching between both states is selectively performed by the switching means. It is therefore possible to simplify the structure of the ion current detector that uses the glow plug having the ion current detecting function, such as wiring of the conductive wires connected to the heating element, and other circuit arrangements associated with ion current detection, and hence to provide an inexpensive ion current detector. In this case, the ion current detection accuracy is never reduced in spite of such simple structure.
For more concrete structure of the ion current detector, the invention as claimed in claim 14 recites that the power source is connected through the switching means to an electric path between the heating element and the wall portion of the combustion chamber, while the invention as claimed in claim 15 recites that the power source is connected directly to the electric path between the heating element and the wall portion of the combustion chamber. Both sufficiently meet requirements for realizing simplification of the structure. However, since the invention as claimed in claim 15 is to apply the voltage between the heating element and the wall portion of the combustion chamber directly without the switching means, it can show the following special effect.
Although the ion current due to fuel combustion is originally weak, since the power supply circuit is constructed without passing through the switching means, the ion current can be detected more precisely. The switching means can be materialized by a switching circuit with plural switch contacts, or a semiconductor switching element (transistor, thyristor or the like), with some resistance thereon.
The power source for applying voltage to the conductive wire pair in the first state and the power source for applying voltage between the heating element and the wall portion of the combustion chamber in the second state may be provided separately as recited in claim 16, or a common power source may be used therebetween as recited in claim 17. In either case, the ion current can be detected precisely. In particular, the invention as claimed in claim 17 does not require an power source exclusively used for the ion current detection, e.g., a power source other than the vehicle battery, thus simplifying the structure.
The invention as claimed in claim 18 recites that one end of the power source is connected to one conductive wire coupled to the heating element while the other is connected to a cylinder head of the Diesel engine for holding the glow plug. In this case, the structure for applying voltage between the heating element and the wall portion of the combustion chamber can be simplified when it is used in the Diesel engine.
The invention as claimed in claim 19 recites that a constant voltage circuit is provided between the power source and one wire of the conductive wire pair for regulating the supply voltage of the power source to a constant value. Since the ion current is originally weak, the ion current value detected is susceptible to variation of the applied voltage to cause a detection error when the applied voltage largely varies. The detection error also causes various problems. For example, when the output information of the ion current (wave height, area, etc.) is used for flame failure detection, the accuracy of the flame failure detection must be lowered. In contrast, the above structure permits the improvement in accuracy of the ion current detection and hence the improvement in accuracy of individual processings such as flame failure detection.
The invention as claimed in claim 20 recites that a plurality of glow plugs are connected in parallel and power-supply paths to individual glow plugs are switched at the same time by the switching means. In such structure, the switching circuit as the switching means and the detecting resistor as the ion current detection means can be shared by the glow plugs, thereby further simplifying the structure. For example, when the glow plugs are provided in the combustion chambers of a multiple cylinder engine, ion current can be detected for each cylinder in time series.
The ion current detector can also be simplified with structure other than the above, and such structure is preferably materialized as recited in claims 21 to 23. The invention as claimed in claim 21 is such that an voltmeter for ion current detection is provided between one conductive wire of the glow plug and the ground contact. In this case, the voltmeter can be constructed by an amplifier measuring a potential difference from the ground with relatively simple structure, rather than by a differential amplifier having relatively complicated internal structure.
In addition, the invention as claimed in claim 21 is preferably constructed as recited in claim 22, i.e., it is preferably constructed such that a capacitor is provided between one conductive wire of the glow plug and the voltmeter. In this case, the DC component of the supply voltage is cut by the capacitor. Therefore, even when a power source of a relatively high voltage (e.g., 50 volts) is used exclusively for the ion current detection, the voltage applied to the voltage detector (amplifier) never exceeds the withstand voltage since the high voltage of the power source is not directly applied to the voltage detector. As a result, inconvenient things such as damage to the voltage detector can be prevented. It should be noted that this structure becomes more effective in the case the supply voltage for the ion current detection is 30 volts or higher.
The invention as claimed in claim 23 is such that an ion current detecting resistor is provided on the grounded side of the power source for detecting ion current from a potential difference between both terminals of the ion current detecting resistor. In this case, the voltage waveform corresponding to the ion current waveform detected is plotted on a reference level of 0 volt. It is therefore unnecessary to use an expensive, complicated voltage detector even when using a supply voltage exceeding the withstand voltage of the voltage detector. Such structure is preferably materialized as recited in claim 16, i.e., it is preferably materialized such that the heating-element power source and the ion current detecting power source are provided separately with the ion current detecting resistor provided on the grounded side of the latter power source. This is because the heating performance of the heating element may be lowered at heating time when the heating element and the ion current detecting resistor are connected in series.
On the other hand, a glow plug as recited in claim 24 can be used for the glow plug in the above ion current detector, in which a heating element portion having a heating element is so provided that it projects into the combustion chamber for burning fuel. An ion current detecting electrode to the inner wall of the combustion chamber is formed in the heating element. In this case, the heating element of the glow plug acts to promote ignition and combustion of fuel in the combustion chamber due to the heating action when the heating element is running hot. When detecting ion current rather than running the heating element hot, the heating element acts as the ion current detecting electrode for detecting the ion current resulting from the fuel combustion. In other words, when detecting ion current, the heating element and the inner wall of the combustion chamber adjacent to the heating element form two electrodes for capturing positive and negative ions existing therebetween when burning the fuel. It is therefore possible to detect the ion current precisely in spite of such simple structure, and hence to effectively use the information on the detected ion current for various combustion control. Further, since the ion current detecting function is given to the glow plug, an inexpensive ion current sensor can be provided.
A glow plug as recited in claim 25 comprises a heating element portion provided with a heat resisting insulator and a heating element embedded in the heat resisting insulator, in which a portion of the heating element is exposed from the heat resisting insulator and the exposed portion is used as an ion current detecting electrode to the inner wall of the combustion chamber. In such a case, since the exposed portion of the heating element effectively acts as the ion current detecting electrode, the same effects as of claim 25 can be obtained, and beside, the following effect can be newly obtained. Although it is considered that some carbon adheres to the exposed portion of the heating element during operation of the glow plug, the adhered carbon is burnt off by the heating action of the heating element (e.g., due to glowing when the engine starts at low temperature). As a result, life of the glow plug is never reduced even in such structure in which an exposed portion is provided in the heating element for use as an ion current detecting electrode, so that the glow plug adds excellent durability that are useful for long periods.
The heating element is preferably made of a ceramic material as recited in claim 26. In this case, if the heating element made of a ceramic material is so arranged that a portion of the heating element will be exposed into the combustion chamber, oxidation wearing of the heating element can be minimized even when it is exposed to hot combustion gases, thereby further improving the durability of the glow plug.
In the invention as claimed in claim 27, the heating-element running state of the glow plug and the ion current detecting state of the glow plug are switched (switching means). In the ion current detecting state of the glow plug, combustion ions are captured between the glow plug electrode portion and the inner wall of the combustion chamber, and ion current is detected by current detection means such as an ion current detecting resistor.
The present invention also features that the switching means is operated to temporarily switch over to the ion current detecting state at least immediately after the fuel ignition stage (operation means). Since the function of the glow plug for promoting ignition and combustion of fuel is given top priority in all the functions of the glow plug, for example, during the afterglow period when the engine starts at low temperature, the ion current detection has not been performed during the afterglow period in the prior art. In contrast, according to the present invention, the ion current detection period is temporarily provided within a range in which the heating function of the glow plug is never damaged even under the heating-element running state such as in the afterglow period. It is therefore possible to detect ion current precisely even in the glow period of the glow plug, and hence to maintain the fuel combustion properly using the result of the ion current detection.
In the invention as claimed in claim 28, the operation means operates the switching means to switch over to the ion current detecting state for a predetermined period of time after each event of the fuel injection into the combustion chamber. In this case, since the ion current detection period is set based on the fuel injection timing, the ion current can be detected securely by setting the ion current detection period as short as possible, thereby minimizing lowering of the glow function of the glow plug.
In the invention as claimed in claim 29, the operation means operates at a predetermined frequency to switch over between the heating-element running state and the ion current detecting state. Even in such a case, the ion current detecting function and the heating-element running function can be united in the afterglow period.
In the invention as claimed in claim 30, the glow plug comprises a heating element energized through a pair of lead wires to generate heat, a heat resisting insulator embedding the heating element therein, and an ion current detecting electrode integrally formed with the heating element. Such a glow plug is used to detect ion current produced when burning fuel. In this case, the ion current can be detected precisely in spite of such very simple structure, and the information on the ion current detected can be effectively used for combustion control.
In the invention as claimed in claim 31, the heating-element running state of the glow plug and the ion current detecting state of the glow plug are switched (switching means). Brief description of the normal switching action is as follows. For example, when the glow plug is held in the heating-element running state at the time of low-temperature start of the engine, and warm-up of the engine is completed by the heating action of the heating element, the glow plug is switched from the heating-element running state to the ion current detecting state. In other words, combustion ions are captured between the exposed electrode portion of the glow plug and the inner wall of the combustion chamber, and the ion current is detected by the current detection means such as an ion current detecting resistor.
The present invention also features that a leakage current flowing from the exposed electrode portion is detected in a predetermined stage before fuel ignition under the ion current detecting state of the glow plug (leakage current detection means). When the leakage current detected by the leakage current detection means is larger than a given threshold, the switching means is operated to temporarily switch over from the ion current detecting state to the heating-element running state (operation means).
The carbon adhered to the outer portion of the glow plug in the combustion chamber can cause a reduction in insulation resistance between the exposed electrode and the grounded portion, and hence a flow of leakage current. In this case, a desired waveform of the ion current may not be obtained. As shown in FIG. 6(b), the leakage current flows before a real waveform of the ion current is plotted (before point A in FIG. 6). In contrast, according to the present invention, the leakage current is detected in a predetermined stage (in the timing period of fuel injection in FIG. 6), so that the carbon adhesion to the outer glow plug can be estimated based on the leakage current. If such a carbon adhesion occurs, the glow plug is changed to the heating-element running state and the adhered carbon is burnt off. It is therefore possible to constantly detect a desired ion current waveform (e.g., the waveform shown in FIG. 24A), and hence to perform ignition detection or flame failure detection precisely using the detection result.
When the carbon adheres to the outer glow plug, the insulation resistance between the exposed electrode and the grounded side depends on the pressure in the combustion chamber. For this reason, as the pressure rises, the insulation resistance is reduced and the leakage current tends to flow. To avoid such an inconvenient thing, the invention as claimed in claim 32 is such that the leakage current is detected when the pressure in the combustion chamber rises. In this case, the presence or absence of the leakage current can be detected securely. The pressure rise in the combustion chamber corresponds to the compression process in the Diesel engine.
The invention as claimed in claim 33 is such that the leakage current is detected in response to the timing of fuel injection into the combustion chamber. For example, the timing period of fuel injection corresponds to a period that elapses between the moment the pressure in the combustion chamber of the Diesel engine rises and the moment just before the fuel burns. It is therefore possible to detect the leakage current more securely under such a condition that the carbon adhered.
On the other hand, the invention as claimed in claim 34 is such that the operation means holds the switching means in the heating-element running state for a period of time according to the leakage current value detected by the leakage current detection means. In other words, the more the carbon adheres to the outer glow plug, the larger the leakage current value will be. It is therefore possible to burn off the adhered carbon securely by setting the hold time of the heating-element running state according to the leakage current value.
The invention as claimed in claim 35, which is dependent on claims 31 to 34, recites that a high-pass filter is provided to the signal output portion of the ion current detector for detecting ion current, and the detection signal is input to the signal processor. According to the claim, since the high-pass filter is incorporated in the system circuitry, the ion current due to combustion can be separated from the leakage current due to a failure of insulation even when the carbon adheres to the ion current detecting electrode of the glow plug, thereby detecting the ion current securely. If the combustion condition information such as ignition stage is judged from the output waveform of the high-pass filter, the judgment processing becomes easy to perform. The inventor""s study confirms that the cut-off frequency of the high-pass filter is preferably set from 50 Hz to 5 kHz, more preferably 100 to 500 Hz.
The invention as claimed in claim 35 can be materialized as described in claims 6 and 7. In the invention as claimed in claim 36, the threshold for use in judging leakage current by the operation means is set to a value near the acceptable maximum value. When the switching means is operated mainly aiming at removal of the adhered carbon, the threshold for use in judging leakage current should be set small. However, the use of the structure as recited in claim 35 permits separation between the leakage current and the ion current even when some leakage current flows. If the threshold for use in judging leakage current is set large within the acceptable range, the number of times the adhered carbon is burnt off will be reduced. It is therefore possible to detect ion current frequently, and hence to detect the combustion conditions frequently.
The invention as claimed in claim 37 comprises comparison means for inputting an output signal of the high-pass filter and comparing the input signal with the threshold for use in detecting combustion conditions. Since the output of the high-pass filter is compared with the threshold for use in detecting combustion conditions, the processing for detecting combustion conditions can be easily carried out.
The invention as claimed in claim 38 is such that the conductive heating-element and the ion current detecting electrode are provided inside the insulator. At least the exposed portion contacting the flame is constructed of the above conductive mixed-sinter with adding a sintering auxiliary made of more than one kind of oxide of rare-earth element. The structure of the mixed sinter is composed of a first crystal phase and a grain boundary phase between the first crystal phases. Portion of the grain boundary phase or the entire grain boundary phase is crystallized into a second crystal phase containing the sintering auxiliary.
The ion current detecting electrode of the present invention is thus composed of the first crystal phase as a crystal phase of either the conductive ceramic material or the nonconductive ceramic material or both, and the grain boundary phase between the crystal phases (FIGS. 56 to 59). The most important feature of the invention is that portion of the grain boundary phase or the entire grain boundary phase is crystallized into the second crystal phase (FIG. 60) while the grain boundary phase in the conventional or typical conductive mixed-sinter are made into glass phase in whole.
The conductive heating-element and the ion current detecting electrode are provided in the insulator in either way that molded parts of them are produced separately, embedded in the ceramic powder material for the insulator and molded integrally, or that the molded parts of the conductive heating-element and the ion current detecting electrode are inserted between two molded parts of insulator produced in advance.
The molded parts of the insulator, the conductive heating-element and the ion current detecting electrode are made up such that main composition of ceramic powders for the molded parts is mixed with paraffin wax and other resin and the mixture is injection-molded.
The ion current detecting electrode is produced of a mixed material that contains a sintering auxiliary made of an oxide or oxides of rare-earth element in addition to the conductive ceramic powder and the nonconductive ceramic powder. As discussed above, the mixed sinter made of such a material is composed of the first crystal phase and the grain boundary phase between the first crystal phases, with portion of the grain boundary phase or the entire grain boundary phase crystallized into the second crystal phase.
As is similar to the ion current detecting electrode, for the conductive heating-element and the insulator, it is also preferable to use such a material that contains a sintering auxiliary made of an oxide or oxides of rare-earth element in addition to the conductive ceramic powder and the nonconductive ceramic powder. The conductive heating-element and the insulator can thus be made in excellent structure with portion of the grain boundary phase or the entire grain boundary phase crystallized into the second crystal phase.
As is similar to the above inventions, the glow plug of the present invention is run hot by the current passing therethrough to promote ignition and combustion of fuel in the combustion chamber, while the ion current detecting electrode form two electrodes with the inner wall of the combustion chamber adjacent to the ion current detecting electrode to detect ion current in the combustion flame. In such structure, the present invention permits precise detection of the ion current and hence the effective use of the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function, it can be manufactured with compact structure and at low cost.
Although some carbon may adhere onto the surface of the ion current detecting electrode during burning fuel, such adhered carbon can be burnt off by the heating action of the conductive heating-element (e.g., due to glowing when the engine starts at low temperature). As a result, the glow plug can detect ion current precisely for long periods.
In the present invention, at least the exposed portion contacting the flame is constructed of the mixed sinter having the above structure. In other words, the structure of the mixed sinter is composed of the first crystal phase and the grain boundary phase between the first crystal phases (FIG. 59), with portion of the grain boundary phase or the entire grain boundary phase crystallized into a second crystal phase containing the sintering auxiliary (FIG. 60).
For this reason, the melting point of the grain boundary phase and the corrosion resistance can be improved more largely than the conventional structure composed of amorphous glass phases without the second crystal in each grain boundary phase. It is therefore possible to improve the performance of the ion current detecting electrode that could resist thermal shock, oxidation and corrosion, and hence to prevent any damage to the ion current detecting electrode, thereby improving reliability of the accuracy in detecting ion current and reliability of the glow plug.
Further, since the glow plug of the present invention is such that the conductive heating-element, the lead wires and ion current detecting electrode are integrally provided inside the insulator, the structure of the glow plug is simplified. It is therefore possible to detect ion current precisely without carbon adhesion and hence to provide a glow plug exhibiting excellent durability without any damage to the ion current detecting electrode.
The content of the sintering auxiliary to the total weight of the conductive ceramic material and the nonconductive ceramic material in the ion current detecting electrode is preferably set in a range from 3 to 25 wt % as recited in claim 39. If less than 3 wt %, the mixed sinter can not improve its compactness and is difficult to form the second crystal phase in each grain boundary phase.
If it exceeds 25 wt %, the grain boundary phase is made into a glass phase without being crystallized. In this case, the melting point of the grain boundary phase is reduced to lower the resistance to thermal shock and corrosion.
The second crystal phase of the ion current detecting electrode preferably exists in each grain boundary phase with a degree of crystallization of more than 5%. If less than 5%, the resistance to oxidation and corrosion can not be so improved since the melting point increases due to existence of the second crystal phase.
As recited in claim 41, the conductive ceramic material is preferably made of more than one kind of the materials such as metallic carbide, nitride and boride. In this case, the second crystal phase can be formed easily.
The ion current detecting electrode provided in the glow plug recited in claim 42 has an exposed portion exposed from the insulator into the flame. The exposed portion also has a ground portion with a surface roughness Rz of 0.1 to 30 xcexcm (an average roughness of 10 points). The surface roughness Rz of the ground portion is an average roughness of 10 points determined under the provision of JIS B 0601, and the value is in a range of between 0.1 xcexcm and 30 xcexcm. If less than 0.1 xcexcm, the ion current can not be detected sufficiently. If more than 30 xcexcm, a crack or cracks may be developed due to thermal shock or the like. The ground portion is controlled within the above range by grinding it with a grindstone or the like. In this case a desired surface roughness Rz is obtained by regulating the grain size of the abrasive of the grindstone and other grinding conditions.
In arranging the conductive heating-element and the ion current detecting electrode in the insulator, molded parts for the conductive heating-element and the ion current detecting electrode are previously produced such as ones shown in FIGS. 62 and 63, embedded in the ceramic powder material for the insulator and molded integrally. Alternatively, the conductive heating-element and the ion current detecting electrode may be inserted between two insulator parts separately produced. The above insulator molded parts or the molded body with the conductive heating-element and the ion current detecting electrode may be made up by an injection molding.
The conductive heating-element and the ion current detecting electrode may also be provided inside the insulator by printing formation. As an example, the printing formation is performed such that two products (green sheets) of ceramic material, e.g., for forming the insulator, are prepared, and the conductive heating-element, the associated lead wires and the ion current detecting electrode are printed on the surface of one product with a conductive material in a desired form by a printing technique such as screen printing, pad printing or hot stamp.
The other product is so stacked that it will cover the printed portion and then firing is performed. The conductive heating-element, the lead wires and the ion current detecting electrode may be printed on two or more products. The conductive heating-element and the ion current detecting electrode may also be printed on different products and laminated together. The insulator with the conductive heating-element, the lead wires and the ion current detecting electrode printed and built therein is thus obtained.
The insulator is cut as required and the ground portion is ground in the exposed portion of the ion current detecting electrode in a manner described above. Such a glow plug as the ground portion of a specific surface roughness Rz is provided in the exposed portion of the ion current detecting electrode can thus be obtained.
As is similar to the above inventions, the glow plug of the present invention having the above structure is operated to generate heat by passing current through the conductive heating-element so that ignition and combustion of fuel in the combustion chamber can be promoted by the heating action. In this case, the ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame. It is therefore possible for the structure of the present invention to detect ion current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function) in the combustion chamber, it can be manufactured with compact structure and at low cost.
In the present invention, the ground portion is provided in the exposed portion of the ion current detecting electrode. The ground portion has a surface roughness Rz ranging from 0.1 to 30 xcexcm. Since the ground portion has lots of micron size irregularities (FIG. 75), electric flux in the electric field between the ion current detecting electrode and the adjacent cylinder head is concentrated to the convexities in the irregularities, and potential gradients become sharp in the neighborhood of the convexities to which the electric flux is concentrated. Such sharp potential gradients attract charged particles of combustion gases into the neighborhood of the convexities. Consequently, the ion current detecting electrode having the ground portion of the specific surface roughness Rz attracts the charged particles in the combustion chamber due to considerable force, thereby further improving the accuracy in detecting ion current.
Further, since in the glow plug of the present invention the conductive heating-element, the lead wires and the ion current detecting electrode are integrally provided inside the insulator, the structure is simplified. It is therefore possible for the present invention to detect ion current precisely without any trouble from carbon adhesion, and hence to provide a glow plug having excellent durability.
The area of the exposed portion provided at the tip of the ion current detecting electrode is preferably set in a range from 1xc3x9710xe2x88x926 to 0.5 cm2 as recited in claim 43. Although the ion current detecting electrode can detect ion output as long as the area (S) of the exposed portion of the ion current detecting electrode is larger than 0, if the area of the exposed portion is less than 1xc3x9710xe2x88x926 cm2, the dimensions of the exposed portion will be very small such as 10 xcexcmxc3x9710 xcexcm or smaller when it is formed by a printing technique, resulting in decreased productivity. On the other hand, if larger than 0.5 cm2, the area occupied by the ion current detecting electrode will become too large and hence the conductive heating-element will be made small, resulting in decreased productivity.
The ion current detecting electrode can be electrically connected to the conductive heating-element as recited in claim 44. In this case, since the ion current detecting electrode and the conductive heating-element can be integrally molded, the manufacturing process becomes simple.
The invention as claimed in claim 45 is made to the glow plug in which the conductive heating-element, the lead wires and the ion current detecting electrode are provided inside the insulator. According to the invention, at least top portion of the ion current detecting electrode is covered with a nonconductive porous layer.
The nonconductive porous layer has a communication hole opening from the surface of ion current detecting electrode into the flame. The porous layer has electrical nonconductivity. The nonconductive porous layer is made up by sintering nonconductive ceramic powder containing a main component such as Si3N4, Al2O3 or SiO2.
As the embodiment shows, the conductive heating-element and the ion current detecting electrode can be provided in the insulator in the following manner: The conductive heating-element and the ion current detecting electrode are previously produced while the insulator having grooves for accommodating them are prepared. The conductive heating-element and the ion current detecting electrode are then embedded in the grooves and integrally baked. The conductive heating-element the ion current detecting electrode and the insulator may be made of ceramic powder.
As is similar to the above inventions, the glow plug of the present invention having the above structure is operated to generate heat by passing current through the conductive heating-element so that ignition and combustion of fuel in the combustion chamber can be promoted by the heating action. In this case, the ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame.
Since the top portion of the ion current detecting electrode is covered with the nonconductive porous layer, the ion current detecting electrode is never exposed to the direct fire of the flame. For this reason, the ion current detecting electrode is not subjected to stress concentration due to thermal shock by the hot flame, and hence any damage such as crack development. Further, since the nonconductive porous layer has the communication hole, ions flow into a space between the ion current detecting electrode and the cylinder head through the communication hole, thereby detecting the ions accurately.
It is therefore possible for the structure of the present invention to detect ion current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function), it can be manufactured with compact structure and at low cost.
Furthermore, since the conductive heating-element is embedded in the insulator, it is never corroded by the combustion flame, so that excellent durability and good heating performance can be displayed for long periods without any reduction in the resistance and changes in heating characteristic. In other words, since the conductive heating-element could resist oxidation wearing, the sectional area is maintained constantly and the resistance is kept at a constant level. Further, the danger of occurrence of inconvenient things such as damage to the conductive heating-element due to a thermal action such as thermal shock in the combustion chamber can be avoided.
Although some carbon may adhere onto the surface of the insulator during burning fuel, such adhered carbon can be burnt off by the heating action of the conductive heating-element (e.g., due to glowing when the engine starts at low temperature). As a result, the glow plug can detect ion current precisely for long periods.
In the glow plug of the present invention, since the conductive heating-element, the lead wires and the ion current detecting electrode are integrally provided inside the insulator, the structure is simplified. It is therefore possible for the present invention to detect ion current precisely without any trouble from carbon adhesion, and hence to provide a glow plug having excellent durability and easy to manufacture.
The thickness of the nonconductive porous layer is preferably between 0.2 mm and 1.5 mm as recited in claim 46. If less than 0.2 mm, damage such as crack development may be caused due to thermal shock by the flame. If more than 1.5 mm, the thickness will become too large and a crack or cracks may be developed due to stress concentration by the hot flame.
The nonconductive porous layer and the insulator are preferably made of the same material as recited in claim 47. In this case, the junction between both is improved, and the resistance to thermal shock is also improved since both has the same coefficient of linear expansion.
The ion current detecting electrode and the conductive heating-element can be combined as recited in claim 48 (FIG. 88) In this case, the conductive heating-element is covered with the nonconductive porous layer at the tip of the glow plug main body.
The ion current detecting electrode can be made of a conductive ceramic material containing MoSi2, WC, TiN or the like, or refractory metal such as W, Mo or Ti.
The top portion of the insulator is preferably formed into a semi-spherical shape. In this case, since the acute angle portion is removed from the tip of the insulator, the turbulence of combustion flame can be prevented in the neighborhood of the ion current detecting electrode to stabilize the detection performance. Further, since thermal stress concentration is prevented, the resistance to thermal shock can also be improved.
The communication hole formed in the nonconductive porous layer can have any hole diameter as long as it is penetrated from the surface of the nonconductive porous layer to the surface of the ion current detecting electrode. For example, the communication hole has only to pass current when the tip of the glow plug is immersed in an alcoholic solution containing water at a ratio of 50 to 50 and a voltage of 12 volt or so is applied between the tip and the solution.
In the invention as claimed in claim 49, the glow plug is so constructed that the ion current detecting electrode will be electrically connected to the midway of the heating element, and that, when R1 denotes electric resistance of a first heating section of the heating element from a first end of the heating element, corresponding to a positive side in passing a DC current through the heating element, to a center of a first connecting portion, at which the ion current detecting electrode is first connected to the heating element; R2 denotes electric resistance of a second heating section of the heating element from the center of the first connecting portion, where a connection between the heating element and the ion current detecting electrode is first established, to a second end of the heating element corresponding to a negative side in passing a DC current through the heating element; and r denotes electric resistance between the first connecting portion and the opening end of the ion current detecting electrode, it will satisfy the relationship of R2 greater than r.
The first connecting portion is a portion at which the ion current detecting electrode is first connected to the conductive heating-element in a path from the positive end to the negative end of the conductive heating-element. Such definition is made by taking into account both a single ion current detecting electrode (FIG. 90) and a plurality of ion current detecting electrodes (FIG. 91) provided for the conductive heating-element. When a plurality of ion current detecting electrodes are provided, the first heating section corresponds a path from the positive end to the closest ion current detecting electrode and the second heating section is a path from the negative end to the adjacent ion current detecting electrode (FIG. 91). In other words, the second heating section may be connected to one or plural ion current detecting electrodes.
To set the relationship between the electric resistance R2 of the second heating section and the electric resistance of the ion current detecting electrode to R2 greater than r, both materials, or width, thickness or length of the conduction path can be changed. As an example of material change, the second heating section and the ion current detecting electrode can be constructed at different mixing rates between the conductive ceramic powder and the nonconductive ceramic powder.
As a material for the conductive heating-element and the ion current detecting electrode, at least one kind of metallic silicide, carbide, nitride or boride such as MoSi2, Mo5 Si3, MoxSi3cy (x=4xe2x88x925; y=0xe2x88x921), MoB, WC or TiN is used. As a nonconductive ceramic material, Si3N4, Al2O3, BN or the like is used. As a sintering auxiliary, more than one kind of oxide of rare-earth element is added.
Hereinbelow, the use of MoSi2 as a conductive ceramic material, Si3N4 as a nonconductive ceramic material and a mixture of Y2O3 and Al2O3 as a sintering auxiliary is shown.
To achieve conductivity, the conductive heating-element and the ion current detecting electrode is constructed by making the grain size of Si3N4 larger than that of MoSi2 so that conductive particles of MoSi2 will be linked together around a nonconductive particle of Si3N4.
Specifically, MoSi2 having a mean diameter of 1 xcexcm and Si3N4 having a mean diameter of 15 xcexcm are used. With the sintering auxiliary, the mean diameter is 1 xcexcm as well. The mixing ratio of MoSi2 to Si3N4 is properly selected within a range of 10-60 to 90-40 (wt%). If the mixing ratio is set as MoSi2:Si3N4=20:80 in the second heating section of the conductive heating-element and as MoSi2:Si3N4 =40:60 ion current detecting electrode, the relationship of R2 greater than r will be achieved. The sintering auxiliary of Y2O3 and Al2O3 is added at a total rate of 10% per weight. As the sintering auxiliary, more than one kind of oxide of rare-earth element other than Y2O3, such as Yb2O3, La2O3 or Nd2O, may be used.
Although the mixture of the conductive ceramic material and the nonconductive ceramic material is used for the conductor, there may be used only the conductive ceramic material or a mixture of the nonconductive ceramic material and metal powder instead of the conductive ceramic material, or only metal powder or a metal wire is also possible.
The insulator is made of a ceramic sinter that is constructed by adding a sintering auxiliary of Y2O3 and Al2O3 to the main composition of conductive ceramic MoSi2 and nonconductive ceramic Si3N4. To achieve nonconductivity, the insulator is constructed by making the grain size of Si3N4 equal to or slightly smaller than that of MoSi2 so that the conductive particles of MoSi2 are surrounded with the nonconductive particles of Si3N4 and divided into parts. Specifically, MoSi2 having a mean diameter of 0.9 xcexcm and Si3N4 having a mean diameter of 0.6 xcexcm can be used.
It is preferable to select an identical or similar mixing ratio among the conductive heating-element, the ion current detecting electrode and the insulator because such a case makes differences small such as in thermal expansion coefficient. As the sintering auxiliary, more than one kind oxide of rare-earth element other than Y2O3, such as one combined with, yttrium, lanthanum and neodymium, may be used.
From the standpoint of heater characteristics of the glow plug, it is also preferable to set the electric resistance R2 of the second heating section in a range of between 0.1xcexa9 and 5xcexa9 and the electric resistance r in a range of between 0.05xcexa9 and 2.5xcexa9.
In arranging the conductive heating-element and the ion current detecting electrode in the insulator, a molded body for the conductive heating-element and the ion current detecting electrode is previously produced such as one shown in FIG. 100, embedded in the insulator and molded integrally. The lead wires are connected simultaneously with this molding process. Refractory metal or its alloy such as tungsten and molybdenum can be used for the lead wires.
Alternatively, the molded body of the conductive heating-element and the ion current detecting electrode may be inserted between two insulator parts separately produced. The above insulator molded parts or the molded body with the conductive heating-element and the ion current detecting electrode may be made up such that main materials of ceramic powders are premixed with a binder of resin and the like, and the mixture is injection-molded. The molded parts are then baked.
The conductive heating-element and the ion current detecting electrode may also be provided inside the insulator by printing formation. As an example, the printing formation is performed such that a product (green sheet) of ceramic material, e.g., for forming the insulator, are prepared, and the conductive heating-element, the associated lead wires and the ion current detecting electrode are printed on the surface of the product with a conductive material by a printing technique such as screen printing, pad printing or hot stamp. The product is then rolled and baked. The insulator with the conductive heating-element, the lead wires and the ion current detecting electrode printed and built therein is thus obtained.
The firing of the injection-molded body or printed body is performed by a hot press method. For example, the body is pressurized at 400 kg/cm2 under one atmosphere of Ar gas and baked at a temperature of 1800 xc2x0 C. for 60 min.
Next, operation and effects of the invention as claimed in claim 49 will be described. The glow plug of the present invention is energized to generate heat by passing current therethrough so that ignition and combustion of fuel in the combustion chamber can be promoted by the heating action. In this case, the ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame.
It is therefore possible to detect ion current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function), it can be manufactured with compact structure and at low cost.
In the present invention, the electric resistance R2 of the second heating section is set larger than the electric resistance r of the ion current detecting electrode. For this reason, when the carbon adhered to the surface of the insulator of the glow plug and caused an electrical short between the ion current detecting electrode and the cylinder head (see FIG. 90), the carbon between the ion current detecting electrode and the cylinder head can be burnt off securely by applying a DC current across the conductive heating-element.
According to the present invention, when burning off the carbon, the DC current flows from the positive end to the cylinder head through the first heating section, the ion current detecting electrode and the adhered carbon since the relationship between the electric resistance R2 of the second heating section of the conductive heating-element and the electric resistance r of the ion current detecting electrode exhibits R2 greater than r. For this reason, the carbon on the surface of the insulator is heated and burnt due to the heat by combination with the air in the combustion chamber. Since the carbon is thus burnt off, the electrical short due to carbon adhesion can be easily eliminated. It is therefore possible to detect ion current accurately for long periods.
Further, since the conductive heating-element is embedded in the insulator, it is never corroded by the combustion flame, so that good heating performance can be displayed for long periods without any reduction in the resistance and changes in heating characteristic. In other words, since the conductive heating-element could resist oxidation wearing, the sectional area is maintained constantly and the resistance is kept at a constant level. Further, the danger of occurrence of inconvenient things such as damage to the conductive heating-element due to a thermal action such as thermal shock in the combustion chamber can be avoided.
In the glow plug of the present invention, since the conductive heating-element, the lead wires and the ion current detecting electrode are provided inside the insulator, the structure is simplified. It is therefore possible for the present invention to detect ion current precisely without any trouble from carbon adhesion, and hence to provide a glow plug having excellent durability.
The electric resistance R2 of the second heating section is preferably set to more than twice the electric resistance r of the ion current detecting electrode as recited in claim 50. In this case, the carbon can be burnt off more securely.
The ion current detecting electrode can be constructed of the main composition of a conductive ceramic material made of more than one kind of metallic silicide, carbide, nitride or boride, or a mixture of a conductive ceramic material and a nonconductive ceramic material as recited in claim 51. In this case, the heat resistance can be improved and the expansion coefficient can be easily adjusted and matched with that of the insulator, so that the resistance to thermal shock can also be improved.
The ion current detecting electrode can also be constructed of the main composition of a material made of one kind of refractory metal having a melting point of 1200xc2x0 C. or higher, or a mixture of a refractory metal material and a nonconductive ceramic material as recited in claim 52. In the former case, since the metal material can be used in the form of wire, the cost associated with material preparation, machining and assembly can be reduced.
In the latter case, the high-temperature resistance and the resistance to oxidation can be improved, and besides, the coefficient of linear expansion can be easily adjusted and matched with that of the insulator, so that excellent durability can be obtained. Since the conductive heating-element of the glow plug is energized to generate heat up to a temperature from 1000 to 1100xc2x0 C., the melting point must be set to 1200xc2x0 C. by taking into account the heat resistance of the ion current detecting electrode.
The exposed portion of the ion current detecting electrode exposed from the insulator preferably has a portion made of more than one kind of noble metal such as Pt, Ir, Rh, Ru and Pd as recited in claim 53. In this case, the resistance to wear and oxidation of the ion current detecting electrode can be improved.
In the invention as claimed in claim 54, the ion current detecting electrode of the glow plug is electrically connected to the midway of the conductive heating-element. The tip of the ion current detecting electrode is exposed from the insulator into the flame, with positioning it more than 2 mm away from the tip of the housing supporting the main body for the insulator and the ion current detecting electrode.
In arranging the conductive heating-element and the ion current detecting electrode inside the insulator, a molded body for the conductive heating-element and the ion current detecting electrode is previously produced such as one shown in FIG. 94, embedded in the ceramic powder material for the insulator and molded integrally. Alternatively, the conductive heating-element and the ion current detecting electrode may be inserted between two insulator parts separately produced. The above insulator molded parts or the molded body with the conductive heating-element and the ion current detecting electrode may be made up by an injection molding.
The conductive heating-element and the ion current detecting electrode may also be provided inside the insulator by printing formation. As an example, the printing formation is performed such that two products (green sheets) of ceramic material, e.g., for forming the insulator, are prepared, and the conductive heating-element, the associated lead wires and the ion current detecting electrode are printed on the surface of one product with a conductive material in a desired form by a printing technique such as screen printing, pad printing or hot stamp.
The other product is so stacked that it will cover the printed portion and then firing is performed. The conductive heating-element, the lead wires and the ion current detecting electrode may be printed on two or more products and laminated together. The conductive heating-element and the ion current detecting electrode may also be printed on different products and electrically conducted in the laminating process or after baked. The insulator with the conductive heating-element the lead wires and the ion current detecting electrode printed and built therein is thus obtained.
As is similar to the above inventions, the glow plug of the present invention having the above structure is operated to generate heat by passing current through the conductive heating-element so that ignition and combustion of fuel in the combustion chamber can be promoted by the heating action. In this case, the ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame. It is therefore possible for the structure of the present invention to detect ion current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function) in the combustion chamber, it can be manufactured with compact structure and at low cost.
In the present invention, the tip of the ion current detecting electrode is located more than 2 mm away from the top portion of the housing. For this reason, even when some carbon is accumulated on the surface of the glow plug main body, the ion current detection can be performed securely. As will be described with respect to FIG. 96, if the distance (L, in FIG. 93) between the tip position of the ion current detecting electrode and the top portion of the housing is less than 2 mm, the detection ratio of ion output will be reduced gradually as the distance becomes short. In contrast, the present invention is to set the distance to 2 mm or longer, so that the ion output can be detected securely.
Such a reduction in the detection ratio with less than 2 mm distance seems to be caused as follows. If the distance (L) between the tip position of the ion current detecting electrode and the top portion of the housing is less than 2 mm, the insulation resistance between the ion current detecting electrode and the housing will be reduced largely to be a simulated short state when the carbon has been accumulated on the glow plug main body. It is therefore difficult to detect ion current. In contrast, since the present invention is to set the distance (L) to 2 mm or longer, the insulation resistance is not so much reduced that a simulated short will occur even when the carbon has been accumulated on the glow plug main body. Even if the insulation resistance is reduced due to long-period operation, the carbon can be burnt off by a hating action caused when the conductive heating-element is electrically conducted as will be described later. It is therefore possible for the glow plug of the present invention to detect ion current securely.
In the glow plug of the present invention, since the conductive heating-element, the lead wires and the ion current detecting electrode 9 are provided integrally, the structure is simplified. It is therefore possible for the present invention to detect ion current precisely without any trouble from carbon adhesion, and hence to provide a glow plug having excellent durability.
When R (xcexa9) denotes the total electric resistance of the conductive heating-element and B (xcexa9) denotes the electric resistance from the positive end of the conductive heating-element to the tip of the ion current detecting electrode, it is preferable to satisfy the relationship of B (xcexa9)xe2x89xa7R (xcexa9)/3 as recited in claim 55. In this case, an optimum current can be passed through a circuit among the conductive heating-element the ion current detecting electrode and the adhered carbon even when the carbon has been so accumulated that a simulated short will occur. For this reason, the carbon can be burnt off by the conductive heating-action of this circuit. After such a simulated short is relieved, the current flows through the conductive heating-element to further promote the carbon burn-off.
If the electric resistance B (xcexa9) is very large, the resistance of the circuit among the conductive heating-element, the ion current detecting electrode and the adhered carbon becomes large. In this case, almost normal current flows through the entire conductive heating-element and the adhered carbon can be burnt off by the heating action of the conductive heating-element even if the adhered carbon exists. It is therefore possible to easily burn and destroy the carbon accumulated on the glow plug main body with maintaining the original heating function of the glow plug constantly.
To set the relationship between R (xcexa9) and B (xcexa9) to B (xcexa9Q)xe2x89xa7R (xcexa9)/3, materials for the conductive heating-element and the ion current detecting electrode, or width, thickness or length of the conduction path can be changed. As an example of material change, the mixing ratio between raw materials of the conductive ceramic powder and the nonconductive ceramic powder is controllable. The length of the conduction path may also be changed by changing the connect position of the ion current detecting electrode to the conductive heating-element.
The invention as claimed in claim 1 is applied to the glow plug constituted of the housing and the main body retained in the housing. The main body includes the insulator; the conductive heating-element provided inside the insulator; the pair of lead wires electrically connected to both ends of the conductive heating-element, drawn out to the outside of the insulator; and the ion current detecting electrode provided inside the insulator for detecting an ionization state in the flame. The tip of the ion current detecting electrode is exposed from the insulator so that it will contact the flame. The glow plug of the present invention features that when K denotes the coefficient of linear expansion of the ion current detecting electrode, H denotes the coefficient of linear expansion of the conductive heating-element and S denotes the coefficient of linear expansion of the insulator, the relationship among them is defined as Hxe2x89xa7S and Hxe2x89xa7K.
If the coefficient of linear expansion H is smaller than S or K, tensile stress will be set up on the surface of the glow plug main body because such compressive stress as will be described later can not be applied thereon. For this reason, there is high possibility of crack development in the glow plug main body to make it difficult to improve the durability of the glow plug.
In arranging the conductive heating-element and the ion current detecting electrode inside the insulator, a molded body for the conductive heating-element and the ion current detecting electrode is previously produced. The molded body is then embedded in the powder material for the insulator and molded integrally. Alternatively, the molded body of the conductive heating-element and the ion current detecting electrode may be inserted between two insulator molded parts separately produced.
The insulator molded parts or the molded body with the conductive heating-element and the ion current detecting electrode may be made up by mixing resin containing main components of the molded body such as ceramic powder and paraffin wax and injection molding the mixture. After that, pressure firing is performed including degreasing, and the baked body is cut to be a ceramic heater with an ion current detecting function.
As is similar to the above inventions, the glow plug of the present invention having the above structure is operated to generate heat by passing current through the conductive heating-element so that ignition and combustion of fuel in the combustion chamber can be promoted by the heating action. In this case, the ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame. It is therefore possible for the structure of the present invention to detect ion current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function) in the combustion chamber, it can be manufactured with compact structure and at low cost.
In the present invention, the relationship among the coefficients of linear expansion H, K and S of the conductive heating-element, the ion current detecting electrode and the insulator is defined as Hxe2x89xa7S and Hxe2x89xa7K. In other words, the conductive heating-element has a coefficient of linear expansion larger than those of the ion current detecting electrode and the insulator. For this reason, compressive stress is maintained on the surface of the glow plug main body when in use. As mentioned above, the glow plug main body is manufactured by molding the powder material and sintering it at a high temperature of about 1800xc2x0 C. Such a sinter is considered not to have any internal stress in a high-temperature state immediately after sintering.
However, since the glow plug is usually used in a range of between room temperature and 100020  C. lower than the sintering temperature, the glow plug main body shrinks compared to the state immediately after sintering. At this time, the relationship among the coefficients of linear expansion H, K and S of the conductive heating-element, the ion current detecting electrode and the insulator is as Hxe2x89xa7S and Hxe2x89xa7K, i.e. the coefficient of linear expansion H of the conductive heating-element embedded inside is larger than the coefficients K and S of the insulator and the ion current detecting electrode exposed on the surface of the main body, so that compressive stress acts on the surface of the main body constantly.
In the present invention, the compressive stress acts on the surface of the glow plug main body constantly when in use. As is well known, such compressive stress can be resistant to damage such as crack development much more than tensile stress. It is therefore possible for the glow plug of the present invention to prevent damage to the main body surface.
Further, since the conductive heating-element is embedded in the rod-like insulator, it is never corroded by the combustion flame, so that good heating performance can be displayed for long periods without any reduction in the resistance and changes in heating characteristic. In other words, since the conductive heating-element could resist oxidation wearing, the sectional area is maintained constantly and the resistance is kept at a constant level. Further, the danger of occurrence of inconvenient things such as damage to the conductive heating-element due to a thermal action such as thermal shock in the combustion chamber can be avoided.
In the glow plug of the present invention, since the conductive heating-element, the lead wires and the ion current detecting electrode are integrally provided inside the insulator, the structure is simplified.
It is therefore possible for the present invention to detect ion current precisely without any trouble from carbon adhesion, and hence to provide a glow plug having excellent durability.
The coefficients of linear expansion K, H and S preferably satisfy the following relationship: Oxe2x89xa6Hxe2x88x92Sxe2x89xa62.0xc3x9710xe2x88x926(/xc2x0C.) and Oxe2x89xa6Hxe2x88x92Kxe2x89xa62.0xc3x9710xe2x88x926(/xc2x0C.).
The case Hxe2x88x92S is less than O is as described above. On the other hand, if Hxe2x88x92S exceeds 2.0xc3x9710xe2x88x926, the tensile stress of the conductive heating-element will become large to cause a sharp rise of the resistance of the conductive heating-element in long-period operation. The case Hxe2x88x92K is less than O is as described above. On the other hand, if Hxe2x88x92K exceeds 2.0xc3x9710xe2x88x926, such a sharp rise of the resistance of the conductive heating-element will also be caused in long-period operation.
The ion current detecting electrode can be constructed of a conductive ceramic material containing the main composition of more than one kind of metallic silicide, carbide, nitride or boride, or a mixture of a conductive ceramic material and a nonconductive ceramic material as recited in claim 58. In this case, the heat resistance can be improved and the expansion coefficient can be easily adjusted and matched with that of the insulator, so that the resistance to thermal shock can also be improved.
The ion current detecting electrode can also be constructed of a refractory metal material containing the main composition of more than one kind of metal having a melting point of 1200xc2x0 C. or higher, or a mixture of the refractory metal material and a nonconductive ceramic material as recited in claim 59. In the former case, since the metal material can be used in the form of wire, the cost associated with material preparation, machining and assembly can be reduced.
In the latter case, the high-temperature resistance and the resistance to oxidation can be improved, and besides, the coefficient of linear expansion can be easily adjusted and matched with that of the insulator, so that excellent durability can be obtained. Since the conductive heating-element of the glow plug is energized to generate heat up to a temperature from 1000 to 1100xc2x0 C., the melting point must be set to 1200xc2x0 C. by taking into account the heat resistance of the ion current detecting electrode.
The invention as claimed in claim 60 is applied to the glow plug including the insulator, the conductive heating-element provided inside the insulator, and the ion current detecting electrode provided inside the insulator for detecting an ionization state in the flame. In the glow plug of the present invention, a conductive layer is provided on the surface of the insulator so as to cover the exposed portion of the ion current detecting electrode exposed from the insulator, with establishing an electrical connection to the ion current detecting electrode.
The conductive layer is provided on an area wider than the area of the exposed portion so that the exposed portion of the ion current detecting electrode exposed from the insulator can be covered with the conductive layer. While the conductive layer is electrically connected to the ion current detecting electrode, the conductive layer has conductivity itself. For this reason, the conductive layer can act to effectively extend the area of the exposed portion of the ion current detecting electrode.
In arranging the conductive heating-element and the ion current detecting electrode in the insulator, a molded body for the conductive heating-element and the ion current detecting electrode is previously produced, such as one shown in FIG. 100, while joining the lead wires thereto. The molded body is then embedded in the ceramic powder material for the insulator and molded integrally. Alternatively, the conductive heating-element and the ion current detecting electrode may be inserted between two insulator parts separately produced.
The above insulator molded parts or the molded body with the conductive heating-element and the ion current detecting electrode may be made up such that the powder materials are mixed with resin containing paraffin wax for its main ingredients and the mixture is injection-molded. Pressure firing is then performed including degreasing, and after that, the baked body is cut to be a cylindrical shape with round tip, thus manufacturing a ceramic heater with an ion current detecting function.
The conductive heating-element and the ion current detecting electrode may also be provided inside the insulator by printing formation. As an example, the printing formation is performed such that a product (green sheet) of ceramic material, e.g., for forming the insulator, are prepared, and the conductive heating-element, the associated lead wires and the ion current detecting electrode are printed on the surface of the product with a conductive material in a desired form by a printing technique such as screen printing, pat printing or hot stamp. The product is then rolled and baked.
The insulator with the conductive heating-element, the lead wires and the ion current detecting electrode built therein is thus obtained. In either technique, the ion current detecting electrode is manufactured to be exposed on the surface of the insulator.
To form the conductive layer on the surface of the insulator, for example, the conditions of the insulator such as shape and roughness are first tailored according to the need. The conductive layer is then printed out on the surface of the insulator in a desired form by a printing technique such as pat printing or a cylinder-screen printing. Other techniques such as plasma coating and evaporation coating can also be used for formation of the conductive layer.
As is similar to the above inventions, the glow plug of the present invention having the above structure is operated to generate heat by passing current through the conductive heating-element so that ignition and combustion of fuel in the combustion chamber can be promoted by the heating action. In this case, the ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame. It is therefore possible for the structure of the present invention to detect ion current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function) in the combustion chamber, it can be manufactured with compact structure and at low cost.
In the present invention, the conductive layer electrically connected to the ion current detecting electrode is provided on the surface of the insulator. For this reason, the conductive layer acts as the exposed portion of the ion current detecting electrode to extend the area of the exposed portion. It is therefore possible to detect ion current more securely and precisely compared to an ion current detecting electrode with no conductive layer, and hence to further improve the fuel control.
In the glow plug of the present invention, since the conductive heating-element, the lead wires and the ion current detecting electrode are integrally provided inside the insulator, the structure is simplified. It is therefore possible for the present invention to detect ion current precisely without any trouble from carbon adhesion while effectively extending the area of the ion current detecting electrode exposed to the flame.
It is preferable to make the insulator partially exposed from the conductive layer so that an edged portion or portions will be formed on the conductive layer as recited in claim 61. In this case, the edged portion displays a tendency to absorb ions (edge effect) compared to the flat portion. For this reason, the ion current detection can be performed in quick response to make it possible, as will be described later, to sharpen the angle of leading edge in the ion current detection stage and to increase the peak value.
As will also be described later, the edged portion formed by partially exposing the insulator from the conductive layer includes not only a case where the conductive layer has a pattern such as a net so that the insulator will be exposed from meshes of the net, but also a case where the conductive layer is made into a solid layer so that the edged portion is formed on the boundary between the solid layer and the exposed portion of the insulator.
The edged portion is preferably rectangle in cross section as recited in claim 62. Such a rectangular-edged portion can be formed in steps without smoothing the boundary with the insulator. In this case, the edge effect can be further amplified.
The conductive layer can be made in net structure so that the insulator will be exposed from meshes of the net (FIGS. 103 to 106) as recited in claim 63. In this case, many rectangular-edged portions can be formed in the meshes and this make it possible to display the edge effect more securely.
The conductive layer can be constructed of metal or conductive ceramic material as recited in claim 64. As such metal, a mixed material of refractory metal and active metal is preferably used. In this case, the active metal improves adhesion of the conductive layer to the insulator while the refractory metal improves the durability.
For such refractory metal, platinum, noble metal such as gold, nickel, steel and chrome are cited, which can be used alone or by mixing them. For such active metal, titanium, zirconium, hafnium and vanadium are cited, which can also be used alone or by mixing them. A preferable combination is of gold and nickel of more than 90 wt % with active vanadium for the remainder. In such a combination, gold and nickel maintain the durability while vanadium improves adhesion to the insulator.
As the conductive ceramic material, various kinds of metallic silicide, carbide, nitride or boride can be used. Silicide is preferably selected in view of the oxidation resistance. It is also preferable to mix an oxide-type ceramic material such as aluminum oxide or silicon dioxide so as to improve adhesion to the insulator.
The thickness of the conductive layer is preferably between 1 xcexcm and 20 xcexcm as recited in claim 65. If less than 1 xcexcm, waves or waste matters due to combustion will collide with or crash the conductive layer to wear it heavily, resulting in loss of durability. The thickness is preferably 5 xcexcm or thicker If more than 20 xcexcm, the thermal expansion coefficient is made largely different from that of the insulator, so that crack development can occur due to thermal changes to cause the conductive layer to come off from the insulator. The thickness is preferably 15 xcexcm or thinner.
The invention as claimed in claim 66 is a modification of the glow plug as claimed in claim 1, in which the insulator includes a first insulating substrate, a covering insulating substrate provided on the front face of the first insulating substrate, and a second insulating substrate stacked on the back face of the first insulating substrate,
the heating element is formed by printing between the front face of the first insulating substrate and the covering insulating substrate, the pair of lead wires are formed by printing between the front face of the first insulating substrate and the covering insulating substrate so as to be connected to both ends of the heating element, and the ion current detecting electrode is provided between the first and second insulating substrates.
Since the conductive heating-element and the lead wires are formed by printing between the front face of the first insulating substrate and the covering insulating substrate while the ion current detecting electrode is provided between the first and second insulating substrates, the glow plug is operated to generate heat by passing current through the conductive heating-element so that ignition and combustion of fuel in the combustion chamber can be promoted by the heating action. In this case, the ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame.
It is therefore possible to detect ion current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function) in the combustion chamber, it can be manufactured with compact structure and at low cost.
In the present invention, since the conductive heating-element is printed and embedded between the first insulating substrate and the covering insulating substrate, it is never corroded by the combustion flame, so that good heating performance can be displayed for long periods without any reduction in the resistance and changes in heating characteristic, thereby improving the durability.
In other words, since the conductive heating-element could resist oxidation wearing, the sectional area is maintained constantly and the resistance is kept at a constant level. Further, the danger of occurrence of inconvenient things such as damage to the conductive heating-element due to a thermal action such as thermal shock in the combustion chamber can be avoided.
Although the ion current detecting electrode may be subjected to carbon adhesion, such adhered carbon can be burnt off by the heating action of the conductive heating-element (e.g., due to glowing when the engine starts at low temperature). As a result, the glow plug can detect ion current precisely for long periods.
In the present invention, the conductive heating-element is formed by printing, e.g., on the front face of the first insulating substrate. Such printing formation is performed according to the following exemplary procedure. The conductive heating-element and the lead wires are formed on the front face of a product (green sheet) with a conductive material in desired forms by a printing technique such as screen printing, pad printing or hot stamp. The product is composed of ceramic material for the first insulating substrate as will be described later. The conductive heating-element and the lead wires can also be formed by printing on the covering insulating substrate.
The second insulating substrate, the first insulating substrate and the covering insulating substrate are stacked in this order. Junction among them is made as will also be discussed in the following 51st embodiment. In other words, the substrates are all made into products of ceramic material, stacked one on another and joined by firing. Alternatively, the substrates may be joined with adhesive.
As discussed above, according to the present invention, the conductive heating-element and the lead wires are formed by printing between the first insulating substrate and the covering insulating substrate. For this reason, the conductive heating-element and the lead wires can be provided inside the glow plug with a thin layered state of between 0.005 mm and 0.02 in thickness, thereby making the glow plug compact. Since the conductive heating-element and the lead wires are never exposed into the combustion flame, the durability of the glow plug can also be improved.
Further, since the conductive heating-element, the lead wires and the ion current detecting electrode are provided integrally together with the covering insulating substrate, the first insulating substrate and the second insulating substrate, the structure is simplified. It is therefore possible for the present invention to detect ion current precisely without any trouble from carbon adhesion, and hence to provide a glow plug having excellent durability.
Each outer surface of the first insulating substrate and the covering insulating substrate can have a curved surface portion as recited in claim 67. In this case, the curved surface portion is used to easily cut the laminated body of the first insulating substrate, the second insulating substrate and the covering insulating substrate to be a cylindrical shape in cross section (see FIG. 4).
The invention as claimed in claim 68 features that the glow plug is formed by stacking the first insulating and the second insulating substrate together with the conductive heating-element, the lead wires connected to both ends of the conductive heating-element and the ion current detecting electrode for detecting an ionization state in the flame being provided therebeteen. In this case, the conductive heating-element, the lead wires and the ion current detecting electrode can be provided in parallel between the first insulating and the second insulating substrate (see 55th embodiment). For this reason, the glow plug can be manufactured easily.
The ion current detecting electrode is preferably formed by printing on the front face of the second insulating substrate as recited in claim 69. In this case, since the ion current detecting electrode is previously formed by printing on the second insulating substrate, the first insulating substrate can be stacked thereon to make the manufacturing process simple.
The ion current detecting electrode is preferably made of a conductive wire and provided between the front face of the second insulating substrate and the back face of the first insulating substrate as recited in claim 70. In this case, since the ion current detecting electrode is previously made into a wire, it can be provided merely by inserting the wire between the first insulating substrate and the second insulating substrate. For this reason, the glow plug can be manufactured easily. For the conductive wire, a metal wire and a sinter of ceramic material may be cited.
The tip of the ion current detecting electrode is preferably exposed at the top portion of the second insulating substrate so as to be exposed into the flame as recited in claim 71. In this case, the responsiveness of ion current detection and the detection accuracy (S/N ratio) can be improved.
The ion current detecting electrode can be made of more than one kind of ceramic material such as MoSi2, WC and TiN as recited in claim 72. In this case, the heat resistance can be improved, and besides, the coefficient of linear expansion can be easily adjusted and matched with that of the insulator, so that the resistance to thermal shock can also be improved.
The ion current detecting electrode can be made of refractory metal containing more than one kind of metals W, Mo and Ti as recited in claim 73. In this case, since the material can be used in the form of wire, the cost associated with material preparation, machining and assembly can be reduced.
The exposed portion of the ion current detecting electrode exposed from the second insulating substrate is preferably provided with more than one kind of noble metal such as Pt, Ir, Rh, Ru and Pd as recited in claim 74. In this case, the ion current detecting electrode can have improved resistance to wear and oxidation.
The top portion of the rod-like insulator is preferably made into a semi-spherical shape as recited in claim 75. In this case, since the acute angle portion is removed from the tip of the rod-like insulator, the turbulence of combustion flame can be prevented in the neighborhood of the ion current detecting electrode to stabilize the detection performance.
The invention as claimed in claim 76 is a modification of the glow plug of claim 1, in which the insulator is a rod-like insulator, the heating element is formed by printing inside the rod-like insulator, the pair of lead wires are electrically connected to both ends of the heating element and drawn out to the outside of the rod-like insulator, and the ion current detecting electrode is provided inside the rod-like insulator with electrical insulation from the heating element established.
The ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame.
It is therefore possible for the structure of the present invention to detect ion current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function) in the combustion chamber, it can be manufactured with compact structure and at low cost.
Furthermore, since the conductive heating-element is printed and embedded inside the rod-like insulator, it is never corroded by the combustion flame, so that good heating performance can be displayed for long periods without any reduction in the resistance and changes in heating characteristic. In other words, since the conductive heating-element could resist oxidation wearing, the sectional area is maintained constantly and the resistance is kept at a constant level. Further, the danger of occurrence of inconvenient things such as damage to the conductive heating-element due to a thermal action such as thermal shock in the combustion chamber can be avoided.
Although the surface of the ion current detecting electrode may be subjected to carbon adhesion during fuel combustion, such adhered carbon can be burnt off by the heating action of the conductive heating-element (e.g., due to glowing when the engine starts at low temperature). As a result, the glow plug can detect ion current accurately for long periods.
In the present invention, the conductive heating-element is formed by printing inside the rod-like insulator. Such printing formation is performed according to the following exemplary procedure. The conductive heating-element and the lead wires are formed on the front face of a product (green sheet) with a conductive material in desired forms by a printing technique such as screen printing, pad printing or hot stamp. The product may be composed of ceramic material for the first insulating substrate. The product is then rolled and baked.
The rod-like insulator with the conductive heating-element and the lead wires printed and built therein can thus be obtained.
On the other hand, the ion current detecting electrode is inserted in and fixed to a hollow portion of the rod-like insulator, formed at the axial center of the product in the rolling process or other stage, through an electrically nonconductive material before firing or after firing.
In the present invention, the conductive heating-element and the lead wires are thus formed by printing inside the rod-like insulator. For this reason, the conductive heating-element and the lead wires can be provided inside the glow plug with a thin layered state of between 0.005 mm and 0.02 in thickness, thereby making the glow plug compact. Since the conductive heating-element and the lead wires are never exposed into the combustion flame, the durability of the glow plug can also be improved.
Further, since the conductive heating-element, the lead wires and the ion current detecting electrode are provided integrally inside the rod-like insulator, the structure is simplified. It is therefore possible for the present invention to detect ion current precisely without any trouble from carbon adhesion, and hence to provide a glow plug having excellent durability.
The invention as claimed in claim 77 is a modification of the glow plug of claim 1, in which the insulator is a rod-like insulator constituted of an electrically insulating core shaft with a hollow portion therein and an insulating substrate covering the outer core shaft, the heating element is formed by printing between the core shaft and the insulating substrate inside the rod-like insulator, the pair of lead wires are electrically connected to both ends of the heating element and drawn out to the outside of the rod-like insulator, and the ion current detecting electrode is inserted in and fixed to the hollow portion of the core shaft with electrical insulation from the heating element established. In this case, since the rod-like insulator is constituted of the core shaft and the insulating substrate, the glow plug can be made easily. The same effects as in claim 76 can also be obtained.
The conductive heating-element is preferably formed by printing on the inside surface of the insulating substrate. In this case, since the conductive heating-element and the lead wires can be previously formed by printing on the insulating substrate in the form of sheet, the glow plug can be manufactured easily because the sheet-like insulator has only to be wound around the core shaft.
The invention as claimed in claim 79 shows a method of manufacturing the glow plug of claim 77, which comprises the steps of preparing the product of the core shaft having the hollow portion and composed of electrically nonconductive ceramic material, and inserting the ion current detecting electrode into the hollow portion,
forming the conductive heating-element and the lead wires on the surface of the product of the insulating substrate composed of electrically nonconductive ceramic material by using a printing technique, placing the product of the core shaft on the printed surface of the insulating substrate and winding the insulating substrate around the outer core shaft, and
heating and baking the core shaft and the insulating substrate. In this case, the glow plug having such effects as discussed in claims 76 and 77 can be easily manufactured.
The invention as claimed in claim 80 is a modification of the glow plug of claim 1, in which the insulator is a rod-like insulator; the heating element is provided inside the rod-like insulator; the pair of lead wires are electrically connected to both ends of the heating element and drawn out to the outside of the rod-like insulator; and the ion current detecting electrode is put in a groove with electrical insulation from the heating element established, the groove provided axially on the outer surface of the rod-like insulator.
As will be described later, the conductive heating-element and the lead wires may be provided inside the rod-like insulator by printing them on the surface of a product (green sheet) of conductive material for the rod-like insulator with a conductive material in desired forms by a printing technique such as screen printing, pad printing or hot stamp. The product is then wound around the core shaft separately produced and the rolled body is baked (see FIGS. 126A through 126D of a 58th embodiment). Alternatively, a laminating method in which an upper sheet with a groove thereon is stacked on the product with the printed portions such as conductive heating-element formed thereon (see FIG. 127 of a 59th embodiment) can be used. The rod-like insulator with the conductive heating-element and the lead wires printed and built therein is thus obtained.
On the other hand, the ion current is inserted in and fixed to a hollow portion of the rod-like insulator before firing or after firing, the hollow portion formed axially on the outer surface of the rod-like insulator.
In the present invention, the glow plug is operated to generate heat by passing current through the conductive heating-element so that ignition and combustion of fuel in the combustion chamber can be promoted by the heating action. In this case, the ion current detecting electrode forms two electrodes with the adjacent inner wall of the combustion chamber to detect an ionization state in the flame.
It is therefore possible to detect ion current precisely and hence to effectively use the information for combustion control. Further, since the glow plug has the ion current detecting function in addition to the original heating function (glow function) in the combustion chamber, it can be manufactured with compact structure and at low cost.
In the present invention, since the conductive heating-element is embedded inside the rod-like insulator, it is never corroded by the combustion flame, so that good heating performance can be displayed for long periods without any reduction in the resistance and changes in heating characteristic, thereby improving the durability. In other words, since the conductive heating-element could resist oxidation wearing, the sectional area is maintained constantly and the resistance is kept at a constant level. Further, the danger of occurrence of inconvenient things such as damage to the conductive heating-element due to a thermal action such as thermal shock in the combustion chamber can be avoided. Furthermore, since the ion current detecting electrode has only to be put in the groove on the rod-like insulator, the glow plug can be easily manufactured.
Although the surface of the ion current detecting electrode may be subjected to carbon adhesion during fuel combustion, such adhered carbon can be burnt off by the heating action of the conductive heating-element (e.g., due to glowing when the engine starts at low temperature). As a result, the glow plug can detect ion current precisely for long periods.
In the present invention, since the conductive heating-element, the lead wires and the ion current detecting electrode are provided integrally inside the rod-like insulator, the structure is simplified. It is therefore possible for the present invention to detect ion current precisely without any trouble from carbon adhesion, and hence to provide a glow plug having excellent durability.
The groove with the ion current detecting electrode therein is preferably filled with a nonconductive coating material so that the ion current detecting electrode can be covered therewith as recited in claim 81. In this case, the ion current detecting electrode can be easily fixed to the rod-like insulator. As such a nonconductive coating material, an electrically nonconductive ceramic material may be used.
The conductive heating-element and the lead wires are preferably formed by printing on the inside surface of the insulator as recited in claim 82. In this case, since the conductive heating-element and the lead wires can be previously formed by printing on the insulating substrate in the form of sheet, the glow plug can be manufactured easily because the sheet-like insulator has only to be wound around the core shaft. Further, since the conductive heating-element and the lead wires can be provided inside the glow plug with a thin layered state of between 0.005 mm and 0.02 in thickness, thereby making the glow plug compact.
The tip of the ion current detecting electrode is preferably exposed at the top portion of the rod-like insulator so as to be exposed into the flame as recited in claim 83. In this case, the responsiveness of ion current detection and the detection accuracy (S/N ratio) can be improved.
The ion current detecting electrode can be made of more than one kind of ceramic material such as MoSi2, WC and TiN as recited in claim 84. In this case, the heat resistance can be improved, and besides, the coefficient of linear expansion can be easily adjusted and matched with that of the insulator, so that the resistance to thermal shock can also be improved.
The ion current detecting electrode can be made of refractory metal containing more than one kind of metals W, Mo and Ti as recited in claim 85. In this case, since the material can be used in the form of wire or plate, the cost associated with material preparation, machining and assembly can be reduced.
The exposed portion of the ion current detecting electrode exposed from the second insulating substrate is preferably provided with more than one kind of noble metal such as Pt, Ir, Rh, Ru and Pd as recited in claim 86. In this case, the ion current detecting electrode can have improved resistance to wear and oxidation.
The top portion of the rod-like insulator is preferably made into a semi-spherical shape as recited in claim 87. In this case, since the acute angle portion is removed from the tip of the rod-like insulator, the turbulence of combustion flame can be prevented in the neighborhood of the ion current detecting electrode to stabilize the detection performance. Further, since thermal stress concentration is prevented, the resistance to thermal shock can also be improved.
The invention as claimed in claim 88 shows a method of manufacturing the glow plug of claim 80, which comprises the steps of forming the conductive heating-element and the lead wires on the surface of the product of the insulating substrate composed of electrically nonconductive ceramic material by using a printing technique,
placing the product of the core shaft of electrically nonconductive ceramic material on the printed surface of the insulating substrate and winding the insulating substrate around the outer core shaft while forming a groove axially among both of rolled-directional end surfaces of the insulating substrate and the core shaft,
arranging the ion current detecting electrode inside the outer groove, and
heating and baking the core shaft and the insulating substrate.
In this case, the ion current detecting electrode can be joined more tightly to the substrate since the width of the substrate narrows due to shrinking of the substrate by firing. In this method, the glow plug having such effects as discussed in claim 80 can be easily manufactured.